Infrared photography with false color materials offers potential value in a wide range of fields. Applications include medical, reconnaissance, geographical surveys, resource management (roads, buildings, and utilities), law enforcement, environmental and agricultural assessment, art authenticity analysis, forgery investigation, and pictorial applications.
Traditionally, false color images are captured photographically with traditional photographic cameras and infrared-sensitive film. The film products available on the market today are: KODAK Aerochrome Infrared Film 2443; and KODAK Aerochrome Infrared NP Film SO-134. These products are sensitive to light in the green (500-600 nm), red (600-700 nm), and infrared (700-900 nm) portions of the electromagnetic spectrum.
It has been proposed to employ 3 monochromatic video cameras, each having a spectral filter (e.g. green red and infrared centered on 550 nm, 650 nm and 850 nm respectively) and aimed at the same scene to produce a false color digital image signal. See the article "An airborne multi-spectral video/radiometer remote sensing system for natural resource monitoring" by C.M.U. Neale, Thirteenth Biennial Workshop on Color Aerial Photography, Orlando Fla., May 6-9, 1992.
More recently, infrared information has been captured with a panchromatic-infrared electronic camera. In this method, a filter wheel is placed before a panchromatic and infrared sensitive charge coupled device (CCD) array in an electronic camera. An image is acquired by sequentially exposing the CCD through a series of filters, which represent the desired spectral bands of the imagery. When imagery is acquired electronically, it can be easily downloaded from the camera into a computer, where it can be analyzed and displayed. Currently, Eastman Kodak's Professional Digital Camera System (DCS) Model 420 IR operates by this sequential filter wheel technique.
Although these technological options exist to capture infrared-sensitive imagery, they are not without problems. Today's infrared-sensitive films are consumable media and require wet photographic processing. Detailed analysis of the resulting images requires photographic scanning for input into geographic analysis computer software or digital image processing routines, causing a considerable delay in preparing and analyzing time-sensitive data. The infrared imaging systems employing video (as opposed to digital) imaging technology suffer from the problems of low resolution, poor response to relative image/camera motion, and the complexity resulting from the use of a number of cameras.
The state-of-the-art digital electronic technology described above requires a dedicated infrared electronic camera, with a moving filter-wheel assembly, as embodied in the DCS Model 420 IR camera system, with the Kodak Color Filter Wheel Assembly. Because the filter wheel requires sequential capture of the imagery bands (it acquires three bands in 40 seconds), the camera can only be used for still photography applications (i.e. where there is no relative movement between the camera and the scene), and is therefore not useful for aerial photography. Also, the filter wheel increases the complexity of the camera system thereby decreasing the mechanical reliability of the system.
Alternatively, an infrared sensitive electronic camera employing beam splitters and three linear detectors is shown in U.S. Pat. No. 4,170,987, issued Oct. 16, 1979 to Anselmo et al. This approach trades off the high cost and mechanical complexity of the filter wheel for the higher cost of three separate image detectors.
From the foregoing it is seen that there is a need for an improved digital electronic infrared camera to capture near infrared imagery.